CHAPTER 8 CELL: THE UNIT OF LIFE 8.1 WHAT IS A CELL? We know that the body of all living organisms is made up of cells which carryout certain basic functions. Hence the cells are called “Basic structural and functional units of living organisms”. The classical branch of biology that deals with the study of structure, function and life history of a cell is called “Cell Biology” • Robert Hooke (1665): He is an English scientist who observed honeycomb like dead cells in a thin slice of cork under microscope. He coined the term ‘cell ‘, which means a small room or compartment • Anton Von Leeuwenhoek (1667): First saw and described living cell • Matthias J Schleiden(1838):a German botanist based on his studies in different plant cells and Theodore Schwann(1839), a British zoologist based on his studies on different animal cells formulated ’cell theory’ 8.2 CELL THEORY: Cell theory was formulated by M J Schleiden (1838) and Theodore Schwann (1839). The main principles of cell theory are • All living organisms are composed of cells and products of cells • All cells arise from pre existing cells through the process of cell division • The body of living organisms is made up of one or more cells 8.3 ORGANISMS SHOW VARIETY IN CELL NUMBER, SHAPE AND SIZE The invention of electron microscope and staining techniques helped scientists to study the detailed structure of cell. Staining Technique: It is a technique of The number of cells vary from a single cell to many cells in an using dyes like eosin, saffranine, organism. The organisms made up of a single cell are called Haematoxylin, fast green, methyleneblue to unicellular organisms. These are capable of independent colour the parts of cells. existence. The single cell carries all the functions like digestion, excretion, respiration, growth and reproduction. So, they are rightly called acellular organisms Eg: Amoeba, Euglena, Paramecium etc. The organisms made up of more than one cell are called multicellular organisms. In multicellular organisms the cells vary in their shape and size depending on their function. The cells are spherical, oval, polyhedral, discoidal, spindle shaped, cylindrical in shape. The shape of the cells varies with the functions they perform. Eg: Parenchyma cells – Polyhedral cells that perform storage function Sclerenchyma cells – Spindle shaped cells that provide mechanical support White blood cells – Amoeboid cells that defend the body against pathogens Nerve cells – Long and branched that conduct nerve impulses Muscle cells – cylindrical or spindle shaped cells concerned with the movement of body parts The size of the cell varies from few micrometers (µm) to few centimeters (cm). The size of bacteria varies from 0.1 to 0.5 µm. The smallest cell PPLO (Pleuro pneumonia like organism) is about 0.1 µ in diameter. The largest cell is an ostrich egg that measures 170 to 180 mm in diameter. Some Sclerenchyma fibres measure up to 60 cm in length. However the average size of the cell ranges from 0.5 to 10 µm in diameter. Units of measurement 1cm = 10mm (millimeter), 1mm = 1000 µm (micrometers), 1 µm = 10000 A0 (Angstrom), 10A0 = 1nm (nanometer) 8.4 CELL STRUCTURE AND FUNCTION: A typical cell has an outer non living layer called cell wall. The cell membrane is present below the cell wall. The cell membrane encloses protoplasm. The protoplasm has a semi fluid matrix called cytoplasm and a large membrane bound structure called Nucleus. The cytoplasm has many membrane bound structures like endoplasmic reticulum, golgibodies, mitochondria, plastids, micro bodies, vacuoles; and non membranous structures like Centrosome and ribosomes. These are called cell organelles. The cytoplasm without these cell organelles is called cytosol. The cytoplasm also contains non living inclusions called ergastic substances and cytoskeleton (microfilaments and microtubules) The content of the cell within cell wall is called protoplast. The cytoplasm without living cell organelles is called cytosol Fig. 8.1 ANIMAL CELL Fig. 8.2 PLANT CELL Comparison of plant and animal cell Plant cell Animal cell Cell wall is present Cell wall is absent Centrioles are absent Centrioles are present Plastids are present Plastids are absent Have large vacuole May have small vacuoles CELL CELL WALL PROTOPLAST Cell membrane Protoplasm Nucleus Cytoplasm Nuclear membrane Cytosol Cell organelles Nucleoplasm Ergastic substances Membrane bound cell organelles Non membranous cell organelles Chromatin Reserve food Endoplasmic reticulum (ER) Smooth ER, Rough ER Ribosomes Nucleolus Secretory products .Golgi complex Centrosome Excretory products Mitochondria Mineral crystals Plastids Cytoskeleton Vacuoles Microtubules Microbodies Lysosomes, Microfilaments Peroxysomes Glyoxysomes 8.5 DESCRIPTION OF THE CELL CONTENTS 1. CELL WALL: It is an outer non living, rigid layer of cell. It is present in bacterial cells, fungal cells and plant cells. It is a permeable membrane chiefly composed of cellulose. It gives rigidity, mechanical support and protection to the cell. Cell wall of bacteria composed of 2. PROTOPLAST: It includes cell membrane and protoplasm. peptidoglycans or murein complex. Cell wall of fungi has chitin. i) CELL MEMBRANE OR PLASMA MEMBRANE: It is a semi permeable membrane present in all cells. It is present below the cell wall in plant cell and outermost membrane in animal cell. It is composed of phospholipids, proteins, carbohydrates and cholesterol. S.J.Singer and G. Nicolson (1974) proposed Fluid Mosaic model to describe the structure of plasma membrane. Fig. 8.3 FLUID MOSAIC MODEL OF PLASMA MEMBRANE Functions: It allows the outward and inward movement of molecules across it. The movement of molecules across the plasma membrane takes place by diffusion, osmosis, active transport, phagocytosis (cell eating) and pinocytosis (cell drinking). ii) PROTOPLASM: It is a living substance of the cell that possesses all vital products made up of inorganic and organic molecules. It includes cytoplasm and nucleus. Purkinje (1837) coined the term protoplasm. Huxley called protoplasm as “physical basis of life” CYTOPLASM: It is the jellylike, semi fluid matrix present between the cell membrane and nuclear membrane. It has various living cell inclusions called cell organelles and non living cell inclusions called ergastic substances and cytoskeletal elements. The cytoplasm without cell organelles is called cytosol. A. MEMBRANE BOUND CELL ORGANELLES PRESENT IN CYTOPLASM 1. ENDOPLASMIC RETICULUM (ER): Discovery: Porter (1945) Endoplasmic reticulum is a network of membrane bound tubular structures in the cytoplasm. It extends from cell membrane to nuclear membrane. It exists as flattened sacs called cisternae, unbranched tubules and oval vesicles. There are two types of ER Rough ER: It has 80s ribosomes on its surface Smooth ER: It does not have ribosomes Fig. 8.4 ENDOPLASMIC RETICULUM Functions: • It helps in intracellular transportation • It provides mechanical support to cytoplasmic matrix • It helps in the formation of nuclear membrane and Golgi complex • It helps in detoxification of metabolic wastes • It is the store house of lipids and carbohydrates 2. GOLGI BODIES / GOLGI COMPLEX / GOLGI APPARATUS / DICTYOSOMES Discovery: Camillo Golgi (1898), an Italian cytologist discovered Golgi bodies in the nerve cells of barn owl. Golgi complex has a group of curved, flattened plate like compartments called cisternae. They stacked one above the other like pancakes. The cisternae produce a network of tubules from the periphery. These tubules end in spherical enzyme filled vesicles. Common name: “Packaging centres” of the cell Fig. 8.5 GOLGI BODIES Functions: • They pack enzymes,proteins,carbohydrates etc.in their vesicles, hence called packaging centres • They produce Lysosomes • They secrete various enzymes, hormones and cell wall material • They help in phragmoplast formation 3. MITOCHONDRIA / CHONDRIOSOME Discovery – Kolliker (1880)- discovered in the muscle cells of insects, Altman called them as Bioplasts, Benda (1897) coined the term Mitochondria Fig. 8.6 MITOCHONDRION Mitochondrion is a spherical or rod shaped cell organelle. It has two membranes. The outer membrane is smooth. The inner membrane produces finger like infoldings called cristae. The inner membrane has stalked particles called Racker’s particles or F0 – F1 particles or Claude’s particle or ATP synthase complex. The mitochondrial cavity is filled with a homogenous granular mitochondrial matrix. The matrix has circular mitochondrial DNA, RNA, 70s ribosomes, proteins, enzymes and lipids. Common name: Power houses of the cell / Storage batteries of the cell Functions: Mitochondria synthesise and store the energy rich molecules ATP (Adenosine triphosphate) during aerobic respiration. So, they are called “Power houses of the cell”. 4. PLASTIDS: Discovery: They were first observed by AFW Schimper (1885) Plastids are present in plant cells and euglenoids. Plastids are classified into three types based on the type of pigments. 1. Chromoplasts: These are different coloured plastids containing carotenoids. These are present in fruits, flower and leaves. 2. Leucoplasts: These are colourless plastids which store food materials. Ex: Amyloplasts: Store starch Aleuronoplasts: Store proteins Elaeioplasts: Store lipids 3. Cholorplasts: These are green coloured plastids containing chlorophylls and carotenoids (carotenes & xanthophylls). Chloroplast is a double membranous cell organelle. The matrix is called stroma. The stroma has many membranous sacs called Thylakoids. They arrange